Process for separating a material from a mixture of mixture which comprises employing a solid water-insoluble, hydrophilic, semi permeable membrane

ABSTRACT

Novel gas separation film membranes are made by grafting alphaolefinic, polymerizable monomers having hydrophilic functional groups and having, e.g., about from 2 to about 12 carbon atoms, to nylon, and forming a film from the resulting copolymers. A complex-forming metal component which is active in the presence of water is provided in the film. The invention also includes the process of separating components from gaseous mixtures using the membranes. Of particular interest is the separation of aliphatically-unsaturated hydrocarbons from mixtures, for example, the separation of ethylene from one or more of ethane, methane and hydrogen.

United States Patent 1 Steigelmann et al.

[ PROCESS FOR SEPARATING A MATERIAL FROM A MIXTURE OF MIXTURE WHICHCOMPRISES EMPLOYING A SOLID WATER-INSOLUBLE, HYDROPHILIC, SEMI PERMEABLEMEMBRANE [75] Inventors: Edward F. Steigelmann; Robert D.

Hughes, both of, Park Forest, 111.

[73] Assignee: Standard Oil Company, Chicago, Ill.

[22] Filed: May 30, 1973 [21] Appl. No.: 365,203

Related US. Application Data [63] Continuation-impart of Ser. No.335,012, Feb. 23,

[52] US. Cl 55/16, 208/308, 260/281.5 C, 260/676 AD [51] Int. Cl B0ld53/23, C07c 11/00 [58] Field of Search 260/677 A, 681.5; 55/16; 208/308[56] References Cited UNITED STATES PATENTS 3/1965 Jolley 55/16 4/1966Cahn 260/677 R 8/1967 Robb et a1. 55/16 11 1 3,865,890 [451 Feb. 11,1975 3,392,209 7/1968 Schneider 260/677 R 3,669,949 6/1972 Yoo 260/680 83,739,003 6/1973 Codet et al. 260/439 3,758,603 9/1973 Steigelmann....260/677 A 3,758,605 9/1973 Hughes 260/677 A 3,770,842 ll/l973Steigelmann.... 2601677 A 3,810,952 5/1974 Durand et al. 260/677 RPrimary ExaminerDelbert E. Gantz Assistant Examiner.luanita M. NelsonAttorney, Agent, or Firm-Morton, Bernard, Brown, Roberts & Sutherland[57] ABSTRACT particular interest is the separation of aliphaticallyunsaturated hydrocarbons from mixtures, for example, the separation ofethylene from one or more of ethane, methane and hydrogen.

8 Claims, No Drawings .HYDROPI-IILIC, SEMI PERMEABLE MEMBRANE Thisapplication is a continuation-in-part of our copending application Ser.No. 335,012, filed Feb. 23, 1973.

This invention relates to hydrophilic, semi-permeable polymer membranescontaining complex-forming metals, comprising copolymers produced bygrafting monomers to nylon polymers. The grafting serves to increase thehydrophilic properties of the nylon polymers. The invention is alsodirected to the preparation of such membranes and to their use inprocesses for separating one or more components from gaseous mix tures.

There is considerable commercial interest in separating components,e.g., aliphatically-unsaturated hydrocarbons, from gas mixturescontaining such materials. The aliphatically-unsaturated hydrocarbonsare reactive materials that serve in various roles, generally asintermediates in chemical syntheses. A number of the unsaturatedhydrocarbons are employed as monomers in the formation of polymers and,in this regard, olefins such as ethylene, propylene, butadiene andisoprene are well known. These olefins, as well as other unsaturatedmaterials, for instance, acetylene, are also used to form relatively lowmolecular weight products.

The aliphatically-unsaturated hydrocarbons are most often made availableon a commercial basis in admixture with other chemical compounds,frequently other hydrocarbons. These unsaturated hydrocarboncontainingstreams are usually by-products of chemical syntheses or separationprocesses. When the hydrocar bon streams are liquid under normalconditions or can readily be made so, ordinary distillation techniquescan be used to separate the hydrocarbon components providing they havesufficiently different boiling points for the process to be economicallyfeasible. Especially when the hydrocarbon mixtures contain materialshaving close boiling points, which is frequently the case withhydrocarbons of the same number of carbon atoms or having a differenceof only one carbon atom, distillation may not be an attractiveseparation procedure. In such cases, more expensive processes are oftenused and involve operations such as solvent extraction or extractivedistillation which entail considerable expense, if indeed they aretechnically feasible in a given situation.

When the mixture containing the aliphaticallyunsaturated hydrocarbon isessentially in a gaseous state at normal or ambient conditions oftemperature and pressure, separation of the desired component from themixture may be even more troublesome. In these situations, cryogenicprocesses may be used, but they are expensive. The components of thesenormally gaseous mixtures may not even have particularly close boilingpoints, but, nevertheless, the mixture must be cooled in order toseparate one or more of its components. In spite of the considerablecost of cryogenic opcrations, the procedure has been employedcommercially for the separation of ethylene from other gaseous materialssuch as ethane and methane.

Our copending application cited above describes methods for separatingmaterials, e.g., aliphaticallyunsaturated hydrocarbons, from mixturescontaining them, and these procedures involve the combined use .ofliquid barrier permeation and metal complexing techniques which canexhibit high selectivity factors. In the processes, the liquid barrieris an aqueous solution containing metal ions which will complex with thematerial to be separated, and the liquid barrier is employed inconjunction with a semi-permeable membrane which is essentiallyimpermeable to the passage of liquid. In preferred systems of this typethe liquid barrier containing the complex-forming metal ions is at leastpartially within a hydrophilic, semi-permeable film membrane. Whenoperating in this manner, there is no need to maintain contact of thefilm with a separate or contiguous aqueous liquid phase during theprocess, thereby facilitating the use of a greater variety ofsemi-permeable members as far as physical configuration is concerned.Thus, the membranes can be designed without the hindrance of having toprovide a separate liquid phase adjacent the film, and this may enablethe use of film configurations having a greater surface or contact area.

In systems for conducting separations in which the aqueous liquid isdisposed as a distinct liquid phase on the feed side of thesemi-permeable membrane, there is the disadvantage of having tointroduce the hydrocarbon mixture into the liquid phase, therebyreducing the effective rate or selectivity of the separation.Alternatively, the aqueous liquid phase has been held in contact with asemi-permeable membrane by absorbing the liquid in a porous solid suchas filter paper, and holding the wet paper next to the semi-permeablemembrane in, for instance, a sandwich-type cell con struction. Thephysical limitations of this system make it unattractive, especiallysince the sandwich construction cannot easily be made in shapes whichafford a sufficiently high surface area of film to provide goodseparation rates. Thus, the flow or separation capacity of these systemsmay make then economically less advantageous than other types ofoperations. The use of semi-permeable membranes having the liquidbarrier within the film overcomes these disadvantages to a considerableextent, and offers high separation rates for a given investment inequipment. Moreover, the latter procedure avoids the necessity formaintaining a separate liquid aqueous phase in the system, and contactof the aqueous phase and the feed mixture containing thealiphatically-unsaturated hydrocarbon to be separated can thereby befacilitated. The film membranes can thus be essentially homogeneousmaterials which are suitable for forming into various shapes, and themembranes may be formed by, for instance, extrusion and can be made intohollow fiber membranes. These fibers are preferred membraneconfigurations because they have the advantages of high surface area perunit volume, thin walls for high transport rates, and high strength towithstand substantial pressure differentials across the membrane andfiber walls. Sandwich membranes cannot readily be fabricated into thehollow fiber form.

In one manner of making the semi-permeable film membranes a film-formingmaterial, such as a nylon polymer, is physically mixed with ahydrophilic polymer, such as polyvinyl alcohol, and the mixture isformed into the desired membrane. Problems arise when using this mannerof forming the films. Since the physically mixed polymer solution cannotbe kept homogeneous while making the films, islands form in the mixture.These islands are simply aggregates of a single polymer which tend toimpart homopolymer properties, as opposed to mixed polymer properties,to certain regions in the membranes. Also, due to the diffusingpenetrant gases contacting the film during prolonged use, some leachingof the hydrophilic polymer may occur, thereby decreasing theeffectiveness of the membrane. These problems have been overcome in thepresent invention by chemically, rather than physically, binding thepolymers by grafting techniques. It has been discovered that monomerscontaining a hydrophilic functional group can be grafted to thefilmforming polyamides, to yield membranes which have increaseduniformity in polymer distribution and exhibit superior efficiency forlonger periods of time,

without adversely affecting the hydrophilic property of I the filmmembrane. The more uniform distribution of the grafted hydrophilicmonomer permits a more uniform and more efficient addition of thecomplexforming metal components to the membrane. The avoidance orreduction of leaching results from the increased hydrophilic nature ofthe membrane which maintains the ionization of the complexing metals.

The membrane films of this invention are preferably produced by forminga mixture of nylon, the monomer to be grafted and the grafting catalystand proceeding with the grafting reaction. The solid product can beseparated by filtration, washed and dried. The film can be made from asolution ofthe grafted copolymer. The film can be subsequently providedwith the complexforming metal component. Variations to this proceduremay be used, for example in the grafting step, as more fully outlinedbelow. Another method comprises first forming a film with the nylon andthen contacting the film with the monomer and catalyst in solution toform the graft copolymer. These methods simplify the process of makingthe membranes commercially by, for example, permitting hot meltextrusion of the copolymer. Since it is difficult to melt two polymerstogether, prior methods included mixing in solvent solutions. The use ofhot melt extrusion casting avoids the use of toxic solvents in themixing and casting operations.

The monomers which can be grafted to the nylon are alpha-olefinic,usually contain from 2 to about 12 carbon atoms, and have one or morehydrophilic functional groups, e.g., active polymerizable vinylmonomers. The monomers may contain more than 12 atoms, especially wherethe monomer contains an abundance of hydrophilic functional groups. Themonomers often consist essentially of carbon, hydrogen, and oxygen, withor without nitrogen and are often, but not necessarily, water-soluble.The addition-polymerizable monomers which may be used include thealcohols, esters, sulfamates, phosphonates, carboxylates and the like,and especially the vinyl alcohols, vinyl esters such as the lower alkylesters of acrylic and methacrylic acids, vinyl ethers, acrylamides, andthe like. The hydrophilic monomer is employed in an amount sufficient toenhance the hydrophilic property of the polyamide, and may, forinstance, be up to about 90 weight percent or somewhat more of thecomposition, based on the total polyamide and monomer. The combined orgrafted monomer component is often at least about 30 weight percent onthis basis, to impart more significant properties to the resulting film.Preferably, the grafted monomer is about 40 to about 75 weight percentof their total weight. The chain length of the polymer grafts to thenylon may be up to about ten or twenty or more monomer lengths, and thegrafts may be formed with, for instance, up to about l5 percent of thetotal nitrogen atoms of the polyamide, often with at least about 1 up toabout 5 or 10 percent of the total polyamide nitrogen atoms.

. The film-forming materials which are employed to provide one componentof the semi-permeable film membranes used in the present invention arethose having a polyamide as an essential constituent. The polyamidefilm-forming materials are generally known and have also been designatedas nylons. These polymers are characterized by having amide groupsserving as recurring linkages between carbon chains in the productstructure, and the polymers may be made by several procedures. Commonly,the polyamides are formed by reacting a polyamine and a dicarboxylicacid or its derivative such as an ester, especially a lower alkyl esterhaving, for instance, about 1 to 4 carbon atoms in each ester group.Other reactions which may be employed to form the polyamides include theself-condensation of monoamino, monocarboxylic acids and the reactionsof cyclic lactams. In any event, the polyamide products containrecurring amide groups as an integral part of the principle polymerchain. The polyamides are described, for instance, in the Kirk-Othmer,Encyclopedia of Chemical Technology, Second Edition, Volume 16,beginning at page 1, lnterscience Publishers, New York, 1968. Among thetypical structural formulas of the linear polyamides are H NRNH(COR')\CONHRNH),,CORCOOH and H NRCO(N- HRCO),,NHRCOOH, where R and R representprimarily carbon-to-carbon chains between functional groups in thereactants, and n represents the degree of polymerization or the numberof recurring groups in the polymer chain. The polyamides which can beused in this invention are generally solid at room temperature, and havea molecular weight which makes then suitable for forming the desiredfilm membranes. Polyamides of this type are described in, for instance,US. Pat. No. 3,355,409.

The carboxylic acids which may be used in forming the polyamides have anacyloxy group (-R-COO) in their structure and the R member of this groupis composed essentially of carbon and hydrogen and often contains about6 to 12 carbon atoms. Such groups may be aliphatic, includingcycloaliphatic, aromatic, or a mixed structure of such types, but thegroups are preferably aliphatic and saturated with respect tocarbonto-carbon linkages. These R groups may preferably have straightchain carbon-to-carbon or normal structures. Among the usefuldicarboxylic acid reactants are adipic acid, sebacic acid, azelaic acid,isophthalic acid, terephthalic acid, and the methyl esters of theseacids.

The polyamies employed in making the polyamide film-forming membranesgenerally have at least two non-tertiary, amino nitrogen atoms. Thesenitrogen atoms may be primary or secondary in configuration, althoughamines having at least two primary nitrogen atoms are preferred. Thepolyamines may also have both primary and secondary nitrogen atoms andthe polyamines may contain tertiary nitrogen atoms. The preferredpolyamines reactants have aliphatic, including cycloaliphatic,structures, and often have from 2 to about 12 carbon atoms. Also, thepreferred polyamines are saturated and have straight-chain structures,although branched-chain polyamines can be used.

Among the useful polyamines'are ethylene diamine, pentamethylenediamine, hexamethylene diamine, diethylene triamine, decamethylenediamine and their N-alkyl substituted derivatives, for instance, thelower alkyl derivatives which may have, for instance, l to 4 carbonatoms in each alkyl substituent.

Preferred grafting techniques involve the use ofa catalyst to initiategrafting. A large variety of grafting catalysts and particularly freeradical graft polymerization catalysts may be used. Ammonium persulfateis of particular interest, as well as organic peroxide catalysts such asbenzoyl peroxide, lauryl peroxide, tertiarybutylhydroperoxide, andcumene peroxide. Other water soluble, free radical-forming catalystsinclude potassium sulfate, sodium sulfate or ammonium sulfate togetherwith sulfur dioxide, alkali metal hydrosulfites, alkali metalpyrosulfites or alkali metal thiosulfates. When using these freeradical-forming catalysts, quantitles of, for instance, about 0.1 to 10,preferably 0.5 to 5%, by weight based on the total weight of the monomermay be used. The temperature range for effecting grafting by using thesecatalysts may be about 20 to 90C., preferably about 40 to 65C.

Preferred catalyst for grafting are the ceric salts. The useful cericsalts may include ceric nitrate, ceric sulfate, ceric ammonium nitrate,ceric ammonium sulfate,

ceric iodate, and the like. Additionally, one may make use of cericdihexyl sulfosuccinate, ceric dioctyl sulfosuccinate. These ceric saltsare preferably dissolved or dispersed in an acidic material, preferablyan inorganic acidic material, prior to use. The pH of the ceric saltacid medium should be 3.5 or below. The acidic material may be, forinstance, sulfuric acid, perchloric acid, nitric acid and the like.These catalyst grafting techniques are more fully described withexamples in US. Pat. Nos. 3,046,078 and 3,557,247, incorporated hereinby reference.

The film membranes of this invention may have a thickness of up to about30 mils. or more. Preferably the thickness is up to about mils. Thefilms are sufficiently thick to avoid rupture during use and generallyhave a thickness of at least about 0.001 mil. The film is sufficientlyhydrophilic to hold the liquid barrier solution at least partly withinthe membrane. This hydrophilic property is present in the film membranedue to the character of both the nylon polymer and the graftedhydrophilic monomer. The hydrophilicity may also be further increased bythe addition of other hygroscopic agents. These optical hydrophilicagents used in addition to and not in place of the grafted monomersinclude, for instance, ethylene glycol, glycerol and propylene glycol.The film membrane may be considered sufficiently hydrophilic to beuseful if it absorbs at least about 5 weight percent of water whenimmersed in distilled water for one day at room temperature andpressure.

In the present invention, the metals which serve in the form ofmetal-containing cations to separate a component from a mixture throughthe formation of metal complexes of desired properties, include, forinstance, the transition metals of the Periodic Chart of Elements havingatomic numbers above 20. Included in these metals are those of the firsttransition series having atomic numbers from 21 to 29, such as chromium,copper, especially the cuprous ion, manganese and the iron group metals,e.g., nickel and iron. Others of the useful complex-forming metals arein the second and third transition series, i.e., having atomic numbersfrom 39 to 47 or 57 to 79, as well as mercury, particularly as themercurous ion. Thus, we may employ noble metals such as silver, gold andthe platinum group, among which are platinum, palladium, rhodium,ruthenium and osmium. The useful base metals of the second and thirdtransition series include, for example, molybdenum, tungsten, rheniumand the like. Various combinations of these complex-forming metals mayalso be employed in this invention, either in the presence or absence ofother non-metal or non-complexing metal components.

The metal is provided in the film or in aqueous liquid barrier of theseparation system in a form which is soluble in this liquid. Thus, thevarious water-soluble salts of these metals can be used such as nitratesand halides, for instance, the bromides and chlorides, fluoborates,fluosilicates, acetates, carbonyl halides or other salts of these metalswhich can serve to formthe desired watersoluble complexes when the filmis in contact with water. The metal salts should not react with anycomponents of the chemical feedstock used in the separation procedure toform an insoluble material which could block the film membrane orotherwise prevent the separation of a component from the feedstock.Also, in a given system, the metal is selected so that the complex willreadily form, and yet be sufficiently unstable, so that the complex willdecompose and the disassociated material leave the liquid barrier,thereby providing a greater concentration of the material to beseparated from the exit side of the membrane than is in the feed. Theconcentration of the metal ions in the film or liquid barrier may berather low and still be sufficient to provide an adequate complexingrate so that excessive amounts of the semi-permeable membrane surfacewill not be needed to perform the desired separation. Conveniently, theconcentration of the complex-forming metal ions in the aqueous solutionforming the liquid barrier is at least about O.l molar and is preferablyabout 0.5 to l2 molar. Advantageously, the solution is less thansaturated with respect to the complex-forming metal ions to insure thatessentially all of the metal stays in solution, thereby avoiding anytendency to plug the film membrane and destroy its permeabilitycharacteristics.

When the complexing ions in the liquid barrier employed in thisinvention include cuprous ions, ammonium ions can be used to providecopper ammonium complex ions which are active to form a complex with thematerial to be separated by the use of the film. We preferably supplyabout equimolar amounts of cuprous and ammonium ions, although eithertype of ions may be in excess. The ammonium ions can be provided invarious convenient ways, preferably as an acid salt such as ammoniumchloride or as ammonium hydroxide or ammonium carbonate. In order toenhance the selectivity of the copper ammonium ion complex in theseparation of this invention, we may also make the film and thus theliquid barrier solution more acidic, by, for instance, providing awater-soluble acid such as a mineral acid, especially hydrochloric acidin the film or liquid barrier solution. Preferably, the pH of the liquidbarrier in this form of the invention is below about 5 with the acid inthe solution. Since silver may form undesirable acetylides withacetylenes, the copper ammonium complex may be a more attractivecomplexing agent when it is desired to use the film to separateacetylenes from various mixtures.

Instead of supplying only a noble metal for complexing the material tobe separated in the process of this invention, we may also employmixtures of noble metal and other cation-providing materials. A portionof the noble metal may be replaced by non-noble metal or ammoniumcomponents. Accordingly, the total of such ion-forming materials in thefilm or in the liquid barrier may be composed of a minor or major amountof either the noblel metal or the non-noble metal, ammonium or othercomponents. Solutions having a major amount of the non-noble metal,ammonium or other cationproviding materials not containing a noble metalwill generally be less expensive, and, accordingly, the noble metal maybe as little as about 10 molar percent or less of the totalcation-providing material in the solution. To reduce expenses at leastabout 10 molar percent, preferably at least about 50 molar percent, on acation basis of the total, a cation-providing material may be other thannoble metal. The non-noble or base metals are preferably of Groups ll toVlll of the Periodic Chart of Elements, and especially those in thefourth and fifth periods, aluminum and magnesium. Zinc and cuprous ionsare preferred ones among these non-noble or base metal components. Thevarious metals may be provided in the liquid barrier in the form of anysuitable compound, such as the acid salt forms mentioned above withrespect to the noble metals.

The amount of water in the liquid barrier employed in this invention maybe a minor portion of the liquid phase, but preferably is a majorportion or even essentially all of the liquid, on a metal salt-freebasis. Thus, small or minor amounts of water, say as little as about 5weight percent, on a salt-free basis in the liquid phase may serve toprovide significant transport for the material to be separated acrossthe liquid barrier. Any other liquid present in the barrier ispreferably watermiscible and should be chosen as not to have asubstantial deleterious effect on the separation to be accomplished. Theliquid barrier may also contain a hygroscopic agent, e.g., in a minoramount, to improve the wetting or hydrophilic properties of the liquidand provide better contact with the feed gas.

In the system of the present invention, the amount of complex-formingmetal in the semi-permeable membrane may vary considerably, but issufficient to accomplish the desired separation. Often, this is a minoramount, say, about 1 to 50 weight percent, of the weight of the membraneon a non-aqueous basis, preferably about 5 to weight percent. A suitableprocedure for placing the solution of complex-forming metal in thesemi-permeable film is by contacting the film with the solution andexerting a differential pressure across the solution and film. Thus, thepressure behind the solution is greater than that on the opposite sideof the film, and as a result, the solution is forced into the film underpressure. Conveniently, the pressure on the solution is aboveatmospheric, and the opposite side of the film is essentially atatmospheric pressure. The

pressure differential need not be large, for instance, it

The membrane containing the complex-forming metal may be handled andtransported in an essentially non-aqueousform or with some watertherein, for instance, an insufficient amount of water to be effectivein the separation. In such case, water would be added to the membrane togive a film bearing sufficient water to be useful in performing theseparation process of the invention. During use of the membrane, theamount of water present is preferably less than that which gives asubstantial distinct or separate aqueous phase on the feed inlet side ofthe membrane. The film membrane can be wetted initially, and if it has atendency to dry during use, additional water can be placed inthe filmwhile it is used on-stream in the separation, for instance, by inclusionof moisture in the gaseous feed charged to the system. Alternatively,but less advantageously, the operation can be stopped for addition ofwater to the film. The water could be used at intervals by stopping thefeeding of the gaseous mixture to the system, and charging water to themembrane at such times. in any event, care should be taken to insurethat the film membrane during use is not so dry that it will exhibitnon-selective permeability to the material to be separated from thefeed, and will thereby not serve to separate a product having anincreased concentration of the desired ingredient.

The film membranes employed in the process of this invention are of theessentially water-insoluble, hydrophilic, semi-permeable type. In theabsence in the film of the liquid containing the complex-forming ions,the film is generally not adequately selective with respect to thepassage of or permeation by the material to be separated to perform thedesired separation at the desired rate. Often, the film is permeable toessentially all of the components in the gaseous feedstock used in thisinvention. However, by having the film contain sufficient aqueous liquidto form a barrier, the simple diffusion of gas through the film isreduced or prevented, and the components of the feed stream must,therefore, traverse the film primarily by becoming part of, and thenbeing separated from, the aqueous liquid phase contained in the film.Thus, in the absence of the complexing metal ion in the aqueous medium,there could be a slight separation effected by the use of water as theliquid medium since the individual components in the gas may exhibitdiffering solubilities in water. In the method of the present invention,however, the selectivity of the separation is greatly increased due tothe presence of the complex-forming metal ions in the aqueous barriermedium. Also, during use in the process of this invention, the film hasa sufficient amount of the aqueous medium so that adequate metal ionsare in solution, or at least react as if they are, to perform thedesired separation.

The film membranes which can be employed in this invention arepreferably self-supporting and have sufficient strength not to requireany additional supporting material on either of its sides during use.With some films, however, it may be necessary, advantageous orconvenient to provide adequate support such as additional film orsheet-like materials on one or both sides of the film membrane. Thesesupporting structures are frequently very thin materials and may bepermeable to both liquids and gases and not serve a separating functionwith respect to any component of the feed stream. Alternatively, thesupporting film may be permeable to gases, but not to liquids.

The metal-containing, semi-permeable films made by the procedure of thepresent invention may be employed, for instance, to separate one or moreunsaturated hydrocarbons by the liquid barrier-complexforming techniquehaving the barrier in the film. Although the aliphatically-unsaturatedhydrocarbon products thus provided may be quite pure materials, forinstance, of greater than 99% purity, the separation procedure may beused merely to provide a significant increase in the concentration of agiven aliphaticallyunsaturated hydrocarbon in a mixture with othercomponents of the feedstock.

The process can be employed to separate variousaliphatically-unsaturated hydrocarbons from other ingredients of thefeed mixture providing at least one of the aliphatically-unsaturatedhydrocarbons exhibits a complexing rate or transfer rate across theliquid barrier in the film that is greater than at least one otherdissimilar or different component of the feedstock. Quiteadvantageously, the system can be used to separatealiphatically-unsaturated hydrocarbons from other hydrocarbons which maybe aliphatically saturated or aliphatically-unsaturated, or fromnonhydrocarbon materials, including fixed gases such as hydrogen. Thefeed mixture may thus contain one or more paraffins, includingcycloparaffins, monoor polyolefins, which may be cyclic or acyclic, andacetylenes or alkylenes, and the mixture may include aromatics havingsuch aliphatic configurations in a portion of their structure. Often,the feed mixture contains one or more other hydrocarbons having the samenumber of carbon atoms as the unsaturated hydrocarbon to be separated oronly a one carbon atom difference. Among the materials which may beseparated according to this invention are ethylene, propylene, butenes,butadiene, isoprene, acetylene and the like.

In the method, the mixture containing the aliphatically-unsaturatedhydrocarbon to be separated may be essentially in the gaseous or vaporphase when in contact with the liquid barrier having dissolved thereinone or more metal ions which form a complex with the unsaturatedhydrocarbon. The liquid barrier can be within and thus in contact withthe semi-permeable membrane which may be permeable to thealiphaticallyunsaturated hydrocarbon-containing mixture in the absenceof the liquid barrier. The membrane can be said to immobilize the liquidbarrier with the membrane. The liquid barrier may in essence becompletely within the semi-permeable structure, and the liquid does notpass from the membrane to an excessive extent under the conditions ofoperation. The membrane is, however, selectively permeable in thepresence of the liquid barrier to the component of the feedstock to beseparated. Since there is little, if any, passage for the feedstockacross the separation zone except by becoming part of or reacting withthe liquid barrier, this liquid barrier controls the selectivity of theliquid barriersemi-permeable membrane combination.

The liquid barrier contains sufficient water and soluble metal ions toform a suitable complex with at least one aliphatically-unsaturatedhydrocarbon component of the feed subjected to the separation procedure.The metal ions readily form the complex upon contact with the feed, and,in addition, the complex dissociates back to the metal ion and analiphatically-unsaturated hydrocarbon component of the complex, underthe conditions which exist on the discharge side of the liquid barrierand semi-permeable membrane as employed in the process. The releasedaliphatically-unsaturated hydrocarbons exit the discharge side of themembrane and can be removed from the vicinity of the barrier and itssupporting structure as by a sweep gas or through the effect of vacuumon this side of the barrier. Thus, the unsaturated hydrocarbon-metalcomplex forms and is decomposed in the complex metal ion-containingliquid barrier, and, as -a result, the material passing through thebarrier is more concentrated with respect to at least onealiphatically-unsaturated hydrocarbon component present in the feedstream.

Often, the reactivity of aliphatically-unsaturated hydrocarbons with thecomplexing metal ions in their order of decreasing activity goes fromacetylenes or dienes to monoolefins, the aliphatically-saturatedhydrocarbons and other materials present being essentially non-reactive.Also, different reactivities may be exhibited among the various membersof a given type of aliphatically-unsatllrated hydrocarbons. The processcan thus be used to separate paraffins from monoolefins, diolefins oracetylenes; diolefins from monoolefins; or acetylenes from paraffins,monoolefins or diolefins; as well as to separate a givenaliphaticallyunsaturated hydrocarbon from another of such materials inits class where the members have differing complexing rates with ortransport rates across the liquid barrier. The feed need only contain asmall amount of aliphatically-unsaturated hydrocarbon, as long as theamount is sufficient so that the unsaturated material to be separatedselectively reacts with the metal complex ions to a significant extent,and thus at least one other component of the feed is less reactive ornon-reactive with the complex-forming metal ions.

The aliphatically-unsaturated materials of most interest with regard toseparation have 2 to about 8 carbon atoms, preferably 2 to 4 carbonatoms. The separation of aliphatically-unsaturated materials fromadmixtures containing other gaseous materials, such as the separation ofethylene or propylene from admixtures with other normally gaseousmaterials, e.g. one or more of ethane, propane, and methane andhydrogen, is of particular importance. Frequently, such feed mixturesfor the process contain about I to 50 weight percent ethylene, about 0to 50 weight percent ethane and about 0 to 50 weight percent methane.Another process that may be of special significance is the separationfrom ethylene or minor amounts of acetylene.

The partial pressure of the aliphatically-unsaturated component of thefeed at the input side of the liquid barrier used in the separation isgreater than the partial pressure of this unsaturated hydrocarbon on thedischarge or exit side of the liquid barrier-semi-permeable membranecomposite. This pressure drop of the unsaturated hydrocarbon to beseparated may often be at least about 0.5 pound per square inch, and ispreferably at least about 20 psi, although the pressure drop should notbe so great that the liquid barrier is ruptured or 0therwisedeleteriously affected to a significant extent. Conveniently, the totalpressure of the feed is up to about 1,000 pounds per square inch. Thedischarge partial pressure of the unsaturated hydrocarbon can preferablybe controlled by subjecting the exit side of the liquid barrier to theaction of a sweep gas that may be essentially inert to forming a complexwith the metal ions in solution in the liquid barrier. The sweep gaspicks up the discharged aliphatically-unsaturated coml 1 ponents, andthe sweep gas may be selected so that it can be readily separated fromthe aliphaticallyunsaturated hydrocarbon material if that be necessaryfor the subsequent use of the unsaturated hydrocarbon. Unless a reactionwith the separated hydrocarbon is desired, the sweep gas should berelatively inert therewith and may be, for instance, butane, carbondioxide or the like.

The temperature across the liquid barrier-semipermeable film compositeemployed in the separation procedure can be essentially constant or itmay vary, and decomposition of the metal-unsaturated hydrocarbon complexcan be affected primarily bythe drop in partial pressure of thealiphatically-unsaturated hydrocarbon on the exit side of the liquidbarrier, compared with the partial pressure on the feed side.Conveniently, the temperature of the liquid barrier may be essentiallyambient, especially in the case of feedstocks that are gaseous at thistemperature and the pressure employed on the feed side of the liquidbarrier. The temperature of the liquid barrier may, however, be reducedor elevated from ambient temperature. Often, the temperature may be upto about 100C., and elevated temperatures may even be desired to put thefeedstock in the gaseous or vapor phase. Neither the temperature nor thepressure used should, however, be'

such as to destroy the difference in transport rate across the liquidbarrier, semi-permeable film composite of the aliphatically-unsaturatedhydrocarbons whose separation is sought, compared with that of the othercomponents of the feed. The conditions should be aal ltp a fi and auq ias ll9w A 10% by weight solution of the grafted copoly mer in methanolis prepared. A film is cast from the solution AgNO solution, for use asa gas separation membrane.

To test the effectivness of the films of the present invention,membranes made .as described above, without saponification and withoutsilver nitrate impregnation, with saponification but withoutimpregnation, and with both saponification and impregnation are tested.

For testing, a closed glass cell is used in which the membrane is placedso as to divide the cell into an inlet and an outlet side. A gas inlettube passes through the cap of the cell and extends into the cell endingnear the membrane. A tube of larger diameter surrounds the inlet or feedtube forming an annular passage which permits exhaust of those gaseswhich do not permeate the membrane. On the outlet side of the cell andmembrane another annular arrangement is used whereby a purging gas,helium, passes into the outlet side of the cell and sweeps away gaseswhich have permeated the membrane. The helium passes in through thesmaller also not be such that physical disruption of the liquid barrieror any other significant malfunction results.

The methods and products of this invention and their value are shownfurther by the following examples. Unless otherwise indicated, thepercentages given are on a weight basis.

3.0 grams of a polyamide (Elvamide 8061 (DuPont) nylon) resin, 150 ml.of 0.01M Ce(NH (SO in 0.2N H 80 and 15 ml. of freshly distilled vinylacetate are added to a flask being purged with argon. The mixture isstirred at room temperature for 15 minutes while the argon purgecontinues. During this time the grafting reaction occurs. The mixture isfiltered and the solids are washed with 2N H 80, and then with water.After drying, the solid is weighed and the grafting efficiency determedto be 46% by weight of the uptake.

The nylon-polyvinyl acetate grafted copolymer can Film Used FeedComposition Nylon-Polyvinyl Acetate Graft Copolymer Nylon-PolyvinylAlcohol Graft copolymer Nylon-Polyvinyl Alcohol Graft copolyme: Ag

Perriteate Composition '(Wt.%) Methane tube and carries away thepermeated or separated gases through the surrounding annular passage.The test cell is divided into upper and lower compartments by locatingthe membrane horizontally across the cell. The cell internalcross-sectional area is 3.8 cm. and the crosssection is fully covered bythe film membrane in a manner to provide an effective membrane area of2.2 cm The main body of the cell has a height of 41 mm. and a gas outletat each end. The feed inlet tube enters the upper end of the cell andopens about 5 mm. above the film, and the sweep gas inlet tube entersthe lower end of the cell and opens about 1 mm. below the film.

A mixed gas of methane, ethane, and ethylene is humidified at F. and issupplied to the cell at 10 ml/min. under a pressure of 30 psig. Thepermeate through the membrane is purged from the cell with a 10 ml/min.stream of helium. The permeate composition and permeation rate aredetermined for each film tested. These results are summarized in Table Ibelow.

Table I Perm. Rate Ethylene Ethane ml/qn'. -min S Fe rme ate As thetable shows, the nylon-polyvinyl acetate graft copolymer and thenylon-polyvinyl alcohol graft copolymer produce about a 50% ethylenepermeate where the feed has approximately this portion of thiscomponent. The nylon-polvyinyl alcohol graft copolymer impregnated withthe silver salt solution produces, however, a greater than 98% ethylenepermeate from the same feed. This difference is vividly exhibited by acomparison of the selectivity, S, which is essentially unity for thefilm having no silver salt, but 65.2 for the film of the presentinvention.

It is claimed:

1. In a method for separating a material from a mixture which comprisescontacting said mixture containing said material with a first side of anessentially solid, water-insoluble, hydrophilic, semi-permeable membranehaving therein aqueous liquid barrier having ions which combine withsaid material to form a watersoluble complex, the partial pressure ofsaid material on a second side of said semi-permeable membrane beingsufficiently less than the partial pressure of said material in saidmixture to provide separated material [Methane Ethane] [Ethylene] Feedonsaid second side of said semi-permeable membrane,

Feed

unsaturated hydrocarbon of 2 to 4 carbon atoms which comprisescontacting a vaporous mixture containing said aliphatically-unsaturatedhydrocarbon with a first side of an essentially solid, water-insoluble,hydrophilic semi-permeable membrane having therein an aqueous liquidbarrier, said semi-permeable membrane being permeable to said vaporousmixture in the absence of said aqueous liquid, said liquid barrierhaving ions which combine with said unsaturated hydrocarbon to form awater-soluble complex, the partial pressure of said unsaturatedhydrocarbon on a second side of said semi-permeable membrane beingsufficiently less than the partial pressure of said unsaturatedhydrocarbon in said vaporous mixture to provide separated unsaturatedhydrocarbon on said second side of said semipermeable membrane, andremoving separated unsaturated hydrocarbon from the vicinity of saidsecond side of said semi-permeable membrane, the improvement whichcomprises employing as said semi-permeable membrane a grafted copolymerfilm consisting essentially of nylon having grafted thereto a polymer ofan alpha-olefinic, hydrophilic, polymerizable monomer.

4. The method of claim 3 wherein said alpha-olefinic, hydrophilic,polymerizable monomer contains from 2 to about 12 carbons atoms andconsists essentially of carbon, hydrogen, and oxygen.

5. The method of claim 4 wherein said monomer is a vinyl monomer andcomprises about 40 to by weight of said copolymer.

6. The method of claim 5 wherein the copolymer is a graft polyvinylalcohol-nylon.

7. The method of claim 6 wherein said ion is noble metal ion.

8. The method of claim 7 wherein said noble metal ion is silver ion.

1. IN A METHOD FOR SEPARATING A MATERIAL FROM A MIXTURE WHICH COMPRISESCONTACTING SAID MIXTURE CONTAINING SAID MATERIAL WITH A FIRST SIDE OF ANESSENTIALLY SOLID, WATER-INSOLUBLE HYDROPHILLIC, SEMI-PERMEABLE MEMBRANTHAVING THEREIN AQUEOUS LIQUID BARRIER HAVING IONS WHICH COMBINE WITHSAID MATERIAL TO FORM A WATER-SOLUBLE COMPLEX, THE PARTIAL PRESSURE OFSAID MATERIAL ON A SECOND SIDE OF SAID SEMI-PERMEABLE MEMBRANE BEINGSUFFICIENTLY LESS THAN THE PARTIAL PRESSURE OF SAID MATERIAL IN SAIDMIXTURE TO PROVIDE SEPARATED MATERIAL ON SAID SECOND SIDE OF SAIDSEMI-PERMEABLE MEMBRANE, AND REMOVING SAID SEPARATED MATERIAL FROM THEVICINITY OF SAID SECOND SIDE OF SAID SEMI-PERMEABLE MEMBRANE, SAIDSEPARATED MATERIAL HAVING A TRANSFER RATE ACROSS SAID LIQUID BARRIER,THAT IS GREATER THAN AT LEAST ONE OTHER COMPONENT OF SAID MIXTURE, THEIMPROVEMENT WHICH COMPRISES EMPLOYING AS SAID SEMI-PERMEABLE MEMBRANE ASEMI-PERMEABLE GRAFTED COPOLYMER FILM CONSISTING ESSENTIALLY OF NYLONHAVING GRAFTED THERETO A POLYMER OF AN ALPHA-OLEFINIC, HYDROPHILIC,POLYMERIZABLE MONOMER.
 2. The method of claim 1 wherein the copolymer isa graft polyvinyl alcohol-nylon.
 3. In a method for separatingaliphatically-unsaturated hydrocarbon of 2 to 4 carbon atoms whichcomprises contacting a vaporous mixture containing saidaliphatically-unsaturated hydrocarbon with a first side of anessentially solid, water-insoluble, hydrophilic semi-permeable membranehaving therein an aqueous liquid barrier, said semi-permeable membranebeing permeable to said vaporous mixture in the absence of said aqueousliquid, said liquid barrier having ions which combine with saidunsaturated hydrocarbon to form a water-soluble complex, the partialpressure of said unsaturated hydrocarbon on a second side of saidsemi-permeable membrane being sufficiently less than the partialpressure of said unsaturated hydrocarbon in said vaporous mixture toprovide separated unsaturated hydrocarbon on said second side of saidsemi-permeable membrane, and removing separated unsaturated hydrocarbonfrom the vicinity of said second side of said semi-permeable membrane,the improvement which comprises employing as said semi-permeablemembrane a grafted copolymer film consisting essentially of nylon havinggrafted thereto a polymer of an alpha-olefinic, hydrophilic,polymerizable monomer.
 4. The method of claim 3 wherein saidalpha-olefinic, hydrophilic, polymerizable monomer contains from 2 toabout 12 carbons atoms and consists essentially of carbon, hydrogen, andoxygen.
 5. The method of claim 4 wherein said monomer is a vinyl monomerand comprises about 40 to 75% by weight of said copolymer.
 6. The methodof claim 5 wherein the copolymer is a graft polyvinyl alcohol-nylon. 7.The method of claim 6 wherein said ion is noble metal ion.
 8. The methodof claim 7 wherein said noble metal ion is silver ion.